Apparatus for accelerating electrons



Feb. 12, 1952 HARTMANN 2,585,549

APPARATUS FOR ACCELERAtIING ELECTRONS Filed Nov. 1, 1950 1N VENTOR A: ATTORNEYJ Patented Feb. 12, 1952 APPARATUS FOR ACGELERATING ELECTRONS Hans Hartmann, Baden, Switzerland, assignor to Aktiengesellschaft Brown, Boveri & Cie, Baden, Switzerland, a joint-stock company Application November 1, 1950, Serial No. 193,498 In SwitzerlandNovember v2, 1949 6 Claims.

The present invention relates to devices for accelerating electrically charged particles such as electrons to high velocity and hence high potential by means of magnetic induction effects. These devices which are known generally on the European continent as ray transformers? and in theUnited States as betatrons are comprised of an evacuated annular tube into which streams of electrons are periodically injected, and an adjacently disposed magnetic system energized -by-alter-nati-ng current which produces a magnetic field varying cyclically with time and which has a spacial distribution such that the injected streams of electrons are accelerated by the field round and round the tube on an orbital path known as the equilibrium circle. The magnetic field divides into two components; one component called the "induction field produces the electron acceleration; the other component called the control field produces an increasing centripetal effect upon the electrons designed to match the increasing centrifugal forcesof the electrons caused by their acceleration along the circular path thereby enabling the electron stream to be con-fined during its entire period of acceleration to an equilibrium circle .of constant position and radius. The electrons which can be injected as themagnetic field produced by the alternating current wave passes through zero accelerate to an extremely high velocity by the time the magnetic field has reached its maximum value and can then be diverted .from the orbit and caused to impinge upon a target to produce X-rays or put to some other use.

In order to maintain the electrons stream on the orbit throughout its entire acceleration phase, which is essential to prevent the stream from possibly striking the wall of the tube and becoming lost, the ratio of the induction field to the control field should satisfy the following relationship throughout the entire acceleration phase =27TT2HT, where is the flux included within the electron orbit, r is the radius of the electron orbit, and Hr is the field strength at the orbit. This condition may be realized by making the reluctance .of the magnetic path greater .by an appropriate amount at the electron orbit than its average reluctance within the orbit. In order to maintain fixed proportionality between the induction flux enclosed by the orbit and the control flux at all times during the acceleration phase, an air gap is usually provided in that part of the magnetic circuit which carries the induction flux within the orbit.

Asa practical matter, in order to be able to mount the annular tube in the air gap between the .annular poles of the magnetic circuit across which the control field passes, a portion of the magnetic structure above the tube must be designed so that it is removable. This means that extreme care must .be exercised .in refitting the removable portion onto the remainder whenever a tube is installed. Despite the extreme .care which is used, .it is of course quite possible that some slight misalignment will occur with .the result that the airgap between the control poles will not be uniform through the circumference.

of the orbit. Moreover it has been found ..most diificult using conventional manufacturing. tech niques to construct .a vbetatron in which the exact ratio of the induction to the control field as set out in the above equation will be obtained.

The principal object of this invention is therefore to provide an improved construction for the magnetic structure of a betatron or similar device whichwill permit adjustment of a portion of the iron in the magnetic circuit after the tube has been installed in order to compen sate for any misalignment of the parts attribut-able to tube changes. or to compensate for any unavoidable faults attributable to practical difiiculties encountered in designing and building a magnetic structure which will produce an induction to control field ratio exactly as expressed by the .foregoing equation.

A more specific object is to provide a magnetic structure for a 'betatron wherein the magnetic iron within the electron orbit which carries the induction fiux includes a pair of inner iron cores disposed respectively on opposite sides of the air gap in the iron and which cores are adjustable longitudinally of the core axis to effect minor compensatory changes in the reluctance of the magnetic. circuit within the orbit. A further object is to provide adjustable induction cores which may be displaced in a direction transverse to the core axis so as to compensate for any slight irregularity in distribution of the control field around the orbit such as may be caused by misalignment of the two confronting masses of magnetic iron which lie at the upper and lower sides of the annular electron tube.

The foregoing as well as other objects and advantages inherent in the invention will become more apparent from the following detailed description of a betatron embodying a preferred form of the invention when taken with the ac companying drawings in which Fig. 1 is a view in top plan, Fig. 2 is a central vertical section taken on .line 'l-.l of Fig. '2 and Fig. 3 is a detail of the upper inner core structure.

Referring now to the drawings, the annular tube It provides therein acircular orbit kalong which the electron stream is accelerated. The tube may include therein both a cathode consti-' tuting asource of the electrons and an anode against which the electron stream is caused to impinge after becoming fully accelerated, "but 1 these have, been omitted inthe interest of simplifying the drawing. Tube It lies between confronting and divergently tapering faces of upper and lower annular control poles II, I2 defining the outer air gap I3 across which the electron guiding control flux component of the magnetic field passes in a direction parallel with the vertical axis :c-r of the device.

The induction fiux component of the magnetic field which produces the eddy field responsible for acceleration of the electron stream passes in a direction parallel with axis 33-11: as indicated by arrows I4 through the central opening of tube I0. Part of this induction flux is carried through upper and lower confronting annular poles IE, IS across an annular air gap I'I' therebetween, and the remainder is carried by upper and lower confronting annular iron cores I8, I9, disposed Within the hollow center of the magnetic structure, across an annular air gap therebetween. The cores I8, I9 thus form auxiliary induction poles.

The exciting winding, which produces both the induction and control flux components of the magnetic field is comprised of upper and lower coils 2| 22 arranged concentric with axis a::c and which surround the upper and lower cylindrical iron bodies 23, 24 that form the control poles and part of the induction pole area. Coils 2|, 22 are connected to a source of alternating current of suitable frequency which may for example be of the order 50 cycles per second although other frequencies have and can be used.

The induction fiux component set up in the induction poles I5, I6, and in the cores I8 and I9. and. the control flux component set up in control poles II, I2 close over a plurality of U-shaped yokes 25 distributed uniformly about the circumference of the cylindrical iron bodies 23, 24 in star arrangement. Eight such yokes are shown in the illustrated embodiment but more or less can be used. Preferably all of the iron bodies constituting the yokes 25, the main induction poles I5, IS, the inner cores I8, I9 and the control poles II, I2 are fabricated from iron laminations 26 arranged vertically and stacked in planes substantially radial to the axis :c-:r. The laminations of the iron bodies 23, 24 forming the main induction poles I5, I6 and the control poles II, I2 are arranged in sector-shaped stacks 21 for convenience in manufacture and assembly, and the inner cores I8, I9 are likewise preferably constituted from a plurality of sector shaped laminated stacks 28 as shown in Fig. 3. Assembly of the iron bodies 23, 24 and the yokes 25 can be in accordance with the principle disclosed in Swiss Patent No. 193,438.

Annular gap I1 between the annular induction poles I 5, I 6 is fixed but according to this invention the axial length of air gap 20 between the confronting end faces of the inner annular cores I8, I9 is made adjustable to the end that the induction flux component can be varied with great precision in relation to the control flux great precision in relation to the control fiux component as may be necessary to arrive at the exact ratio therebetween which is required to maintain the electron stream on the desired orbit is. As will be seen from Fig. 1, the inner confronting faces of cores I 8, I9 are separated from one another by a plurality of spacing pieces 29, 30 of nonmagnetic material located substantially in the plane of tube III, ie in the plane of orbit is, and are anchored in position by means of a screw bolt 3| which extends through the axial opening 32 in cores I8, I9.

A lower pressure plate 33 longer than the diameter of the central axial opening 34 within the lower iron body 24 so as to overlap and rest upon a marginal portion of this opening and through which bolt 3| passes, forms an anchor for the lower headed portion 3Ia of bolt 3| and also a seat and pressure member for the bottom end of core I9. A similar plate 35 overlying a marginal portion of the axial opening 33 in iron body 23 serves as an anchor for the upper nut 3| b end of bolt 3| as illustrated, the thickness of the non-magnetic spacers 29, 30 (as measured in the direction of axis x-zr) being such that sufiicient endwise pressure can be exerted on the assembly of inner cores I8, I9 and the spacing pieces 29, 30 therebetween to hold the same against accidental displacement.

In order to change the width of air gap 20 to effect an adjustment of the induction fiux all that need be done is to remove upper core I8 and replace the spacing pieces 29, 30 with others which are thicker or thinner depending upon the I sense of the adjustment required, thereby effecting a relative axial displacement between the inner, confronting end faces of cores I8, I9. 'A thicker spacer will increase the distance between the ends of cores I8, I9 and hence increase the length of gap 20 thereby effecting an increase in reluctance and a decrease in induction flux, while use of a thinner spacing piece will have the opposite effect on the induction flux. If spacing pieces considerably thinner than those illustrated should be required, it is obvious that take-up washers (not illustrated) may have to be used between the outer end of at least one of the cores I8 or I9 and the associated plate 35 or 33 so as to enable the core and spacer assembly to be held in end-wise compression as the bolt nut 3Ib is screwed down. For only slightly thinner spacing pieces, the resiliency of pressure plates 33, 35 will be sufiicient to clamp the inner core assembly to the iron bodies 23, 24.

As previously explained, distribution of the control fiux component which passes across the air gap I3 between the confronting faces of annular control poles I I, I2 at orbit k should be uniform all around the orbit so that the strength of this flux will be the same at any point on the circumference of the orbit. Otherwise, the electron stream will not receive the uniform stabilization which is required. It may well be however that in repacing the upper iron boyd 23 of the magnetic structure after a change of tube II], the workman may fail to align the upper body 23 exactly with the lower body 24 resulting in a non-uniformity of the control flux component at some sector of the orbital path k. The present invention also preferably provides means for compensating for any such non-uniformity in distribution of the control flux component.

To this end, the present invention provides for shifting the inner core structure, which is normally arranged concentric with the energizing coils 2 I, 22, when the upper and lower bodies 23, 24 of the magnetic structure are in perfect coaxial alignment, parallel to itself in a direction transverse to the axis of coils 2 I, 22. While both the upper and lower inner cores I3, I9 could be arranged for such transverse movement to a position eccentric of the axis of the exciting coils, satisfactory compensation for non-uniform distribution of the control flux may be obtained by leaving the lower core I9 in fixed position concentric with the axis of the lower coil 22 and providing a transverse displacement only for the upper core I8 eccentrically of the upper coil 2|.

For this purpose it will be seen that four adjusting screws 31 arranged radially of the axis at-zc are provided at the upper end or" the magnetic structure, the screws being carried by angle brackets 38 secured upon the upper face of the iron body 23. Screws 31 are arranged 90 apart and the screw ends extend radially inward for contact with the upper, tined ends 39 of the sleeve 40 which fits tightly over the upper end of core l8. Such construction enables the upper core [8 to be adjusted parallel to itself as shown in the drawing to the degree of eccentricity (relative to the axis of coil 2i) necessary to restore the distribution of the control flux to uniformity. The central opening 32 through core i8 is of course made sufiiciently greater than the diameter of bolt 3! to permit an appreciable lateral adjustment of core l8, and for the same reason, the central opening 36 through the upper iron body 23 will be larger than the external diameter of core 18.

If desired, the amiular gap 4| between cores [8, I9 and the iron bodies 23, 24 can be used to blow a current of air through the interior of the magnetic structure for cooling purposes.

As previously explained, all parts of the magnetic structure including the adjustable inner core are fabricated from iron laminations arranged on edge substantially radial to the vertical axis x:c of the device. As a result of such axial symmetry, and assuming perfect vertical alignment of the iron bodies 23, 24, the magnetic flux tends to distribute itself uniformly in all radial directions from the axis .r-r. I-lence any displacement of the movable core [8 in any radial direction in compensation for misalignment expresses itself in the same manner, thus assuring a reliable reciprocal adaptation of the induction and control fluxes.

Finally it should be noted that the individual sectors of laminations which make up the iron bodies 23 and 24 actually would have an opening angle much smaller than that shown in the drawing which was done to simplify the latter.

In conclusion, while a preferred embodiment of the invention has been described and illustrated, it is to be understood that the specific construction shown is to be regarded as typical rather than limitative of the scope of the invention as expressed by the appended claims. while the invention has been described with respect to its application to a betatron type of accelerator, it is also applicable to other types of accelerators such as the synchrotron which has a like magnetic structure and operates at least over part of the electron accelerating phase on the same principle as the betatron, namely by magnetic induction effects.

I claim:

1. In a device for accelerating electrons by magnetic induction, an annular tube providing therein a closed orbital path along which the electrons are accelerated, a pair of substantially concentrically arranged annular induction poles disposed in fixed spaced relation at the central opening through said tube and which establish a fixed air gap between the confronting annular end faces thereof substantially in the plane of said tube for flow of induction flux through the tube opening, a pair of auxiliary induction cores disposed respectively within the central openings of said induction poles, the end faces of said cores also confronting each other in spaced relation to establish a second air gap therebetween substantially in the plane of said tube, one of said cores Moreover,

being movable longitudinally of the other to adjust the distance between the end faces of said cores and thereby effect a corresponding adjustment in the length of said second air gap, and coil means energizable from a source of voltage variable with time surrounding said induction poles and disposed substantially concentric therewith.

2. A device for accelerating electrons as defined in claim 1 wherein said auxiliary induction cores are cylinders having a diameter somewhat smaller than the diameter of the central opening through said annular induction poles to establish a radial gap therebetween, and means carried by one of said induction poles for shifting one of said cores transversely to a position eccentrically of said coil means.

3. A device for accelerating electrons as defined in claim 2 wherein the means for shifting one of said auxiliary cores includes a sleeve surrounding an end portion of the core and a plurality of adjusting screws disposed radially about and engageable with the periphery of said sleeve.

4. A device for accelerating electrons as defined in claim 1 wherein said auxiliary induction cores are also annular and which further includes non-magnetic spacer means between the confronting faces of said cores, a bolt extending through said cores and pressure plate means on said bolt cooperative with the outer end faces of said induction poles and cores for clamping said cores in position to said induction poles.

5. A device for accelerating electrons as defined in claim 1 wherein said induction poles and cores are constituted by laminations arranged substantially radially to the axis thereof.

6. In a device for accelerating electrons by magnetic induction, an annular tube providing therein a closed orbital path along which the electrons are accelerated under the combined action of a magnetic induction flux and a magnetic control flux whose respective field strengths vary with time, a magnetic structure associated with said tube for providing said induction and control fluxes, said structure comprising a pair of substantially concentric cylindrical control poles disposed in confronting relation adjacent the upper and lower sides of said tube at said orbital path through which said control flux passes, substantially concentric upper and lower annular induction poles isposed radially inward from said control poles providing a path for flow of the induction flux through the central opening within said annular tube, said induction poles confronting one another in fixed and spaced relation to establish a fixed air gap therebetween substantially in the plane of said tube, upper and lower auxiliary induction cores disposed respectively within the central openings of said upper and lower annular induction poles, said cores also confronting each other in spaced relation to establish a second air gap therebetween substantially in the plane of said tube, said induction cores being movable longitudinally within said induction poles to adjust the distance between said cores and thereby effect a corresponding adjustment in the length of said second air gap, and coil means arranged substantially concentric with and surrounding said induction and control poles adapted to be energized from a voltage varying with time to produce the flow of induction and control fluxes in said magnetic structure.

HANS HARTMANN.

No references cited. 

